This invention relates to the casting of metal strip. It has particular but not exclusive application to the casting of ferrous metal strip.
It is known to cast metal strip by continuous casting in a twin roll caster. Molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term xe2x80x9cnipxe2x80x9d is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip. This casting pool may be confined between end closure side plates or dams held in sliding engagement with the ends of the rolls.
In twin roll casting, eccentricities in the casting rolls can lead to strip thickness variations along the strip. Such eccentricities can arise either due to machining and assembly of the rolls or due to distortion when the rolls are hot possibly due to non-uniform heat flux distribution. Specifically, each revolution of the casting rolls will produce a pattern of thickness variations dependent on eccentricities in the rolls and this pattern will be repeated for each revolution of the casting rolls. Usually the repeating pattern will be generally sinusoidal, but there may be secondary or subsidiary fluctuations within the generally sinusoidal pattern. By the present invention these repeated thickness variations can be very much reduced by imposing a pattern of speed variations in the speed of rotation of the rolls. Compensation in this manner is possible because even small speed variations vary the time of contact of the solidifying metal shells on the rolls within the casting pool and therefore the thickness of the shells which are brought together at the nip. It is thus possible to compensate for an increase in the nip tending to produce a thickening of the strip by an instantaneous acceleration of the rolls so as to decrease the time for shell solidification thereby to produce a compensating tendency for thinning of the strip. Furthermore, varying solidification time will result in varying casting roll temperature distribution which will result in roll shape change and when appropriately matched with initial roll eccentricity will compensate for it.
According to the invention there is provided a method of casting metal strip comprising introducing molten metal between a pair of chilled casting rolls forming a nip between them to form a casting pool of molten metal supported on the rolls and confined at the ends of the nip by pool confining end closures, rotating the rolls so as to cast a solidified strip delivered downwardly from the nip, transporting the strip away from the nip, inspecting the strip as it is transported away from the nip to determine a pattern of thickness variations along the strip due to eccentricities of the casting roll surfaces, and imposing a pattern of speed variation on the rotation of the casting rolls determined by said pattern of thickness variations so as to reduce the amplitude of the thickness variations.
Said pattern of thickness variations may be a regularly repeating pattern.
Preferably, the strip is inspected by an inspection means which produces signals indicative of the frequency and amplitude of repeating thickness variations and the speed of the casting rolls is varied in accordance with those signals.
The pattern of imposed speed variations may comprise a single variation for each revolution of the casting rolls. Alternatively, there may be more than one variation for each revolution of the casting rolls.
Preferably, the rolls are rotated by electric drive motor means and the pattern of imposed speed variations is imposed by feeding said signals directly to the drive motor means.
The imposed speed variation may be applied at an initial timing phase relative to the rotation of the rolls and the phase then varied to minimise the amplitude of the thickness variations.
The method of the invention may also include the step of varying the average speed of rotation of the rolls throughout the cast to maintain a constant average thickness of the strip.
The invention further provides apparatus for casting metal strip comprising
a pair of parallel casting rolls forming a nip between them;
a metal delivery system for delivering molten metal into the nip to form a casting pool of molten metal supported above the nip;
a pair of pool confining end closures disposed one at each end of the pair of casting rolls;
roll drive means to rotate the rolls in opposite directions to deliver a cast strip downwardly from the nip;
strip transport means to transport the strip away from the nip;
strip inspection means to inspect the strip as it is transported away from the nip to determine a pattern of thickness variations along the strip due to eccentricities of the casting roll surfaces; and
control means to impose a pattern of speed variations on the rotation of the casting rolls determined by said pattern of thickness variations so as to reduce the amplitude of the thickness variations.
Preferably, the inspection means is operable to generate signals indicative of the frequency and amplitude of the thickness variations and the control means is effective to control operation of the roll drive means in response to those signals.
Preferably, the roll drive means comprises electric motor means and the control means is effective to feed said signals to the electric motor means.